In copending application Ser. No. 529,338 filed on Sept. 6, 1983 by Martin Edward Wacha and Jerry William Digney there is disclosed a writing instrument in which the barrel also functions as the reservoir which contains the ink supply. For esthetic and other reasons the barrel is relatively thin, making it difficult to attach a pocket clip to it. Moreover, since the rear end of the barrel must be open during the ink filling operation, it is desirable to close that opening during the final assembly operation.